The chance to book tax-free gains before the New Year stands out amid the fog over whether Congress will allow a slate of tax increases to take effect in 2013. Tax-free is good no matter what happens next. If lawmakers do nothing, the 0% rate on net long-term gains for certain investors will rise to 10%.

Taxpayers with capital gains that fall in the bottom two brackets can qualify for the tax bonanza.

Among those who stand to benefit: retirees, couples with one spouse who earns far more than the other, and families with adult children. Some already have taken advantage of this break since it took effect in 2008. Others are learning about it now.

"This is an often missed opportunity," says Robert W. Stanley, a financial adviser in Libertyville, Ill. Even when people know of the 0%, rate, he says, they often fail to realize how many taxpayers qualify.

Jim Holtzman, an adviser in Pittsburgh, says people often are incredulous when he tells them they can sell an asset and owe no tax. A retired executive with whom Mr. Holtzman works has sold some stock in the company he works for to book gains without paying tax. Another is weeding the stocks of various companies from a brokerage account and paying no tax on the gains he's cashing in.

Pass It On

One father funded his daughter's wedding by making her a gift of appreciated stock, which she then sold without having to pay any tax, says Benjamin E. Birken, an adviser at Woodward Financial Advisors Inc. in Chapel Hill, N.C.

ENLARGE

Ross MacDonald

Couples can get the low rate even if one spouse is in a top bracket. A man with a far greater income than his wife, for example, can transfer an asset to her, and she can sell it and get the 0% rate. The two, of course, must file separate tax returns.

"You have to do it very carefully," said Melanie Lauridsen, a technical manager at the American Institute of CPAs. Transferring assets from one person to another is a legal move that requires numerous steps.

Still, a lot of couples use the strategy to great success, though most don't have quite the colorful profile of one pair Ms. Lauridsen helped. The woman in this case had around $450 million in liquid assets; her husband earned somewhere around $40,000 a year (before tax deductions that brought his taxable income lower). She regularly transferred assets to him, and he sold them with no capital-gains tax due.

A caveat: Don't hand over an asset to a spouse for the tax break unless you are truly prepared to relinquish control. Once in the name of someone else, it belongs to them.

ENLARGE

Andrew Altfest, an adviser in New York, has made it a year-end project to identify clients who can benefit from the 0% rate. About a quarter of his clients are in the lower brackets. His firm will comb their portfolios for stocks that have performed well but make sense to sell.

One client who qualifies has $20 million in inherited wealth. The man's taxable income is low because he has many deductions, largely for charitable contributions, and he has put money into trusts that keep assets out of his estate.

"I would not be surprised if many advisers did not know their clients' exact tax brackets," says Mr. Altfest.

Traps and Restrictions

Anyone who wants to qualify can't have more than $35,350 of taxable income excluding capital gain, or $70,700 for a married couple filing jointly. The portion of capital gain that takes taxable income up to this threshold gets the 0% rate; remaining gains over the threshold are taxed at 15%.

As ever with taxes, the 0% rate has hidden traps. State income tax is one: A person who books gains may owe nothing in federal tax on them but be on the hook at the state level.

Also, capital gains can expose more of a retiree's Social Security income to tax.

Children subject to the "kiddie tax" on unearned income above a certain amount (in general, those 19 or younger and full-time students under age 24) don't get the 0% rate.

And there are three exceptions where capital gains are taxed at higher rates: gains from the sale of qualified small-business stock, collectibles such as coins or art, and previously depreciated property legally defined as real property.

Corrections & Amplifications Taxpayers who have some capital gains that fall within the bottom two brackets can get the 0% rate on that portion of them on 2012 returns. Gains over the threshold of $35,350 in taxable income ($70,700 for a married couple filing jointly) are taxed at 15%. An earlier version of this article incorrectly implied that the 0% rate would not be available for any capital gain when overall taxable income exceeds the threshold. Also, the maximum amount of net capital losses that can be deducted from ordinary income is $3,000 on an individual or joint tax return and $1,500 for a married individual filing separately. An earlier version of this article included an information box that incorrectly indicated the $1,500 figure was for an individual filer.

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